1886, | The Limits of Organic Evolution. 417 
has an infinite possibility in its lines of specialization. A simple 
spherical mass of cells, the supposed common ancestor of the, 
animal kingdom, may be modified in a very great variety of direc- 
tions, each of which may give rise to a different type of animal. 
This possibility lies in the fact that it is as yet yndifferentiated 
and unspecialized. But just as soon as it does become modified 
in any one direction the possibilities decrease. Some of the de- 
scendants of this ancestor becoming vertebrates are forever pre- 
cluded from becoming anything else ; others becoming mollusks 
must remain mollusks forever, with all of their-descendants. And 
as later descendants become further modified in any direction into _ 
definite types, the chance for future modification, becomes rapidly 
less. It is only the absolutely undifferentiated which has infinite 
possibilities, for as soon as a single step is taken in any direction 
they become finite. Now it is. plain, since evolution does not 
retrace its steps, that with every step in advance the possible lines 
of development become less and less. All the descendants of the 
vertebrate line must conform to the vertebrate type. The verte- 
brate becomes separated into fish, reptile and mammal, and the 
individual of each group is still further fettered in its develop- 
ment by the special line which its ancestors have taken. e 
descendants of the animals which have started the order of birds 
can not take any new line. They can develop to perfection this 
type, but there they must stop. And so on, with every advanced 
. Step the possibilities of expansion are constantly decreasing.’ 
_ Now a continued specialization of this sort is sure to reach a 
limit eventually, it must run to extremes and then stop. Devel- 
opment must reach a position where further advance is no longer 
possible. Let us illustrate this principle by a concrete exam- 
ple. A five-toed appendage is an unspecialized form which we 
may conceive as modified in many directions. It may becomea = 
grasping organ or a supporting organ or a swimming organ, etc. — ; Be 
In the group to which our ruminants belong this appendage has 
become a supporting structure. In this same group there has : 
further been a tendency to rise upon the toes, in such a manner — 
- that instead of walking on the soles of the feet and palms of the 
-~ hands, the animals in question walk more and more upon the 
fingers and toes. When this peculiarity first began to manifes 
itself, the mammals had five toes. As it became more and mo: 
Pe 
This idea can be found fully expressed in the writings of Professor Cope- 
